Heat storage material

ABSTRACT

A heat storage material which comprises CaCl 2 .6H 2  O, as a main component, containing at least one anhydride selected from the group consisting of an anhydrous bromide and an anhydrous nitrate.

The present invention relates to a heat storage material which utilizes a latent heat of fusion at a temperature of about 20° C. The heat storage material is useful for a heating system such as an agricultural vinyl house designed to utilize a solar energy.

A heat storage material is a substance whereby a latent heat of fusion or solidificaiton in its phase change is utilized for storing or radiating heat. Such a heat storage material is useful for an air-conditioning, a waste heat recovery or a solar heat storage.

CaCl₂.6H₂ O is considered to be a promising heat storage material, since it has a phase change temperature of 29° C. and a latent heat of fusion of 41 cal/g and is inexpensive. Further, the present inventors have earlier proposed a technique to prevent crystallization of CaCl₂.4H₂ O (the melting point of the α-phase: 45° C.) which is likely to take place during repeated phase changes of CaCl₂.6H₂ O. (For instance, see Japanese Unexamined Patent Publication No. 38879/1982, Japanese Unexamined Patent Publication No. 95597/1982 which corresponds to U.S. application Ser. No. 295147 and Japanese Unexamined Patent Publication No. 185377/1982 which corresponds to U.S. application Ser. No. 332172.)

However, CaCl₂.6H₂ O has a melting point of 29° C., which is slightly too high for the utilization of a solar energy, particularly when the solar energy is to be utilized in a vinyl house during winter time so that the energy stored during the daytime is to be radiated during the night time. Thus, CaCl₂.6H₂ O is not adequately practically useful for such a purpose. The optimum phase change temperature for such a purpose is considered to be from 20° to 25° C.

On the other hand, the present inventors have proposed to mix CaCl₂.6H₂ O with a proper amount of an appropriate hydrate to lower the melting point to about 20° C. (Japanese Unexamined Patent Publications Nos. 43387/1976, 76183/1976 and 128052/1976). For instance, it is known that a practically useful heat storage material having a melting point of about 20° C. can be obtained by adding to CaCl₂.6H₂ O about 20 molar % of e.g. MgBr₂.6H₂ O or CaBr₂.6H₂ O. However, such a mixture is costly, since it is necessary to use a relatively large amount of a hydrate of calcium bromide or magnesium bromide. Thus, the mixture has a substantial economical disadvantage that its cost is as high as 5 to 10 times the cost of a heat storage material made of CaCl₂.6H₂ O alone.

Accordingly, it is an object of the present invention to overcome the above-mentioned disadvantage of the conventional heat storage materials, and to provide a practically useful heat storage material having a melting point of about 20° C.

It has been found that the above object can be attained by adding to CaCl₂.6H₂ O at least one anhydride selected from the group consisting of an anhydrous bromide (such as KBr or NH₄ Br) and an anhydrous nitrate (such as KNO₃ or NH₄ NO₃).

Thus, the present invention provides a heat storage material which comprises CaCl₂.6H₂ O, as a main component, containing at least one anhydride selected from the group consisting of an anhydrous bromide and an anhydrous nitrate.

Now, the present invention will be described in detail with reference to the preferred embodiments.

Firstly, the addition of the anhydrous bromide will be described. Table 1 shows the melting points and the heat of fusion of samples in which KBr or NH₄ Br is incorporated. Further, for the purpose of comparison, similar data are given with respect to samples in which a chloride, i.e. KCl or NH₄ Cl, is incorporated.

                  TABLE 1                                                          ______________________________________                                         Melting points and heat of fusion of samples                                   (when an anhydrous bromide is added)                                                         Concentra-                                                                               Melting                                                                               Heat of                                                       tion (% by                                                                               point  fusion                                          No.  Additives                                                                               weight)   (°C.)                                                                          (cal/g)                                                                              State at 30° C.                    ______________________________________                                         1    KCl      5         28     39.5  Uniform liquid                                                                 phase                                     2    KCl      10        27     40.7  KCl slightly                                                                   precipitated                              3    KBr      5         25     38.8  Unifrom liquid                                                                 phase                                     4    KBr      10        24     38.6  KBr slightly                                                                   precipitated                              5    NH.sub.4 Cl                                                                             2         24     40.7  Uniform liquid                                                                 phase                                     6    NH.sub.4 Cl                                                                             5         26     43.5  NH.sub.4 Cl                                                                    precipitated                              7    NH.sub.4 Cl                                                                             8.3       25     37.3  NH.sub.4 Cl                                                                    precipitated                              8    NH.sub.4 Br                                                                             2         27     41.4  Uniform liquid                                                                 phase                                     9    NH.sub.4 Br                                                                             5         23     36.4  Uniform liquid                                                                 phase                                     10   NH.sub.4 Br                                                                             10        22     35.0  NH.sub.4 Br                                                                    slightly                                                                       precipitated                              ______________________________________                                    

It is evident from Table 1 that KCl and NH₄ Cl are inferior in their ability to lower the melting point, and they are not suitable as the additive. Whereas, KBr and NH₄ Br are found to provide desirable effects.

The saturation point in the solubility of these additives is at a concentration slightly lower than 10% by weight, since a small amount remains undissolved at the concentration of 10% by weight. The amounts of addition of KBr and NH₄ Br are practically at most 10% by weight, preferably about 5% by weight, when they are added alone, respectively.

Now, the addition of an anhydrous nitrate will be described. Table 2 shows the melting points and the heat of fusion of samples in which two nitrates, i.e. KNO₃ and NH₄ NO₃, are added alone or in combination with their bromides, i.e. KBr and NH₄ Br, respectively.

                                      TABLE 2                                      __________________________________________________________________________     Melting points and heat of fusion of samples                                   (when an anhydrous nitrate is added)                                                               Melting                                                                             Heat of                                                           Concentration                                                                          point                                                                               fusion                                                No.                                                                               Additives                                                                               (% by weight)                                                                          (°C.)                                                                        (cal/g)                                                                            State at 30° C.                            __________________________________________________________________________     11 KNO.sub.3                                                                                5      21   32.7                                                                               Uniform liquid phase                              12 KNO.sub.3                                                                               10      17   24.3                                                                               KNO.sub.3 slightly precipitated                   13 KNO.sub.3 :KBr                                                                          2:2     23   35.0                                                                               Uniform liquid phase                              14 NH.sub.4 NO.sub.3                                                                        2      22   33.0                                                                               Uniform liquid phase                              15 NH.sub.4 NO.sub.3                                                                        5      15   24.0                                                                               Uniform liquid phase                              16 NH.sub.4 NO.sub.3                                                                       10       8   18.5                                                                               NH.sub.4 NO.sub.3 slightly pre-                                                cipitated                                         17 NH.sub.4 NO.sub.3 :NH.sub.4 Br                                                          1:5     21   34.5                                                                               Uniform liquid phase                              18 NH.sub.4 NO.sub.3 :NH.sub.4 Br                                                          2:5     21   32.3                                                                               Uniform liquid phase                              19 NH.sub.4 NO.sub.3 :NH.sub.4 Br                                                          2:2     20   36.9                                                                               Uniform liquid phase                              __________________________________________________________________________

It is evident from Table 2 that both KNO₃ and NH₄ NO₃, like the above-mentioned bromides, exhibit desirable effects. However, when compared with the case where the bromides are added, not only the decrease of the melting point is slightly greater, but also the decrease of the heat of fusion is correspondingly greater. Accordingly, the practical amounts of addition are rather limited as compared with the case of anhydrous bromides. From Table 2, it is seen that both KNO₃ and NH₄ NO₃ have a saturation point in their solubility at a concentration of about 10% by weight. Thus, the practical amount of addition is at most 10% by weight, but it is preferably from 2 to 3% by weight.

Further, from the experimental results in Table 2 with respect to Samples No. 13 and Nos. 17 to 19, it is evident that when these bromides and nitrates are added in combination, the stability in the phase change is further improved over the case where they are added independently. For instance, when NH₄ NO₃ and NH₄ Br are added each in an amount of 2% by weight, Sample No. 19 has a melting point of about 20° C. and a heat of fusion of about 37 cal/g, thus indicating that the decrease of the heat of fusion is not so great as compared with the substantial decrease of the melting point. This composition is also advantageous from the economical point of view, since the amounts of the additives may be small. Besides, the cost of NH₄ NO₃ is substantially lower than the cost of NH₄ Br. Thus, this composition is substantially less costly, for instance, as compared with the case where NH₄ Br is added in an amount of 5% by weight. Similar advantageous effects are obtainable also in the case where KNO₃ and KBr are added each in an amount of 2% by weight (Sample No. 13 in Table 2). With respect to the case where the anhydrous nitrate and the anhydrous bromide are added in combination, the upper and lower limits of the total amount will be mentioned. Firstly, the upper limit of the amount of the addition of the anhydrous nitrate is determined on the basis of its solubility (i.e. at most 10% by weight) and the allowability of the decrease in the heat of fusion. However, the upper limit is usually determined primarily based on the latter factor. For instance, if the allowability of the decrease in the heat of fusion is set to be about 10 cal/g, the upper limit for KNO₃ is about 5% by weight and the upper limit for NH₄ NO₃ is about 2% by weight (see Table 2). Likewise, the upper limit of the amount of addition of the anhydrous bromide itself is determined on the basis of its solubility (i.e. at most 10% by weight) and the degree of the decrease in the melting point. However, the upper limit is usually determined primarily based on the degree of the decrease in the melting point. For instance, as shown in Table 1, the upper limit for KBr is about 5% by weight and the upper limit for NH₄ Br is likewise about 5% by weight.

Accordingly, when the anhydrous nitrate and the anhydrous bromide are added in combination, it may be said that the practical upper limit of the total amount is, for instance, 10% by weight in the case of a combination of KNO₃ and KBr, or 7% by weight in the case of a combination of NH₄ NO₃ and NH₄ Br. The lower limit of the total amount of these additives should be determined depending upon the desired degree of the decrease in the melting point of the composition, and should eventually be evaluated also from the viewpoint of the costs which constitute the most important factor for the evaluation. Accordingly, it may be said that the upper limit of the total amount of the additives is 10% by weight and the lower limit is about 2% by weight. However, from the practical point of view, a total amount of about 5% by weight is most preferred.

Nitrates and bromides other than those mentioned above, for instance NaNO₃ and NaBr, do not have a solubility as high as the solubility of the potassium salt or the ammonium salt, and accordingly they do not have equal effectiveness. LiNO₃ and LiBr are superior in their effectiveness to the corresponding sodium salts, but from the economical point of view, they can not be regarded as practical additives, since they are very expensive.

Further, potassium salts or ammonium salts other than those of the nitrates or bromides mentioned here, for instance sulfates, acetates, carbonates, hydrocarbonates, phosphates or fluorides, are not practically useful as the additives, since they react with CaCl₂.6H₂ O to readily form insoluble calcium salts.

As described in the foregoing, according to the present invention, at least one anhydride selected from the group consisting of an anhydrous nitrate (such as KNO₃ or NH₄ NO₃) and an anhydrous bromide (such as KBr or NH₄ Br) is added to CaCl₂.6H₂ O, whereby the melting point of CaCl₂.6H₂ O can be lowered. The compositions thereby obtained have superior phase change stability and has a feature that the overall cost of the additives is low. These compositions will be described in detail in Examples 1 to 6 given hereinafter.

The above-mentioned compositions are per se useful as heat storage materials which can be practically used for an extended period of time. However, they have more or less a disadvantage that they tend to have a concentration gradient. It has been found that when at least one hydrate selected from the group consisting of a chloride, a bromide and a nitrate is further added in a small amount to the above-mentioned compositions, the formation of the concentration gradient and the super cooling can be prevented and heat storage materials having improved solidification characteristics and phase change stability can be obtained.

Namely, among the above-mentioned compositions, CaCl₂.6H₂ O containing two anhydrides i.e. KNO₃ and KBr, or NH₄ NO₃ and NH₄ Br, in an amount of 5% by weight each, is useful as the composition of the invention described below. When at most 10% by weight of at least one hydrate selected from the group consisting of e.g. Ca(NO₃)₂.4H₂ O, CaBr₂.6H₂ O, Mg(NO₃)₂.6H₂ O, MgCl₂.6H₂ O and MgBr₂.6H₂ O is added to such a composition, the concentration gradient in the vertical direction (i.e. the direction of the gravity) in the liquid is substantially eliminated, and it is thereby possible to obtain a heat storage material which is free from crystallization of undesirable CaCl₂.4H₂ O. The amount of addition of the above-mentioned hydrate may be determined taking the costs into account, and a combination suitable for a practical purpose may be selected within a range of the total amount of at most 10% by weight, preferably at most 5% by weight. The primary purpose for the addition of these hydrates is to prevent the formation of the concentration gradient. Accordingly, it is practically most important to obtain the maximum effect with the minimum cost (the minimum amount of the addition). If these hydrates are added excessively, it is likely that the melting point is further lowered to a level which is outside the desirable range of the melting point. For instance, the hydrate of a nitrate has greater effectiveness to reduce the melting point as compared with the hydrate of a halide, and when 10% by weight of the hydrate of a nitrate is added to CaCl₂.6H₂ O, the melting point will be lowered to a level of about 20° C. When the same amount of the hydrate of a nitrate is added to the above-mentioned composition, the melting point will be lowered to a level of about 10° C., which is too low for practical purposes. In the case of the hydrate of a halide, the decrease of the melting point is not so great. However, in the case of the halide of a bromide, the decrease of the melting point is fairly great, and its cost is rather high. Therefore, the upper limit is about 10% by weight. As shown in Table 3, the lower limit is 1% by weight in the case of the halide of a nitrate, and may be slightly higher (from 1 to 3% by weight) in the case of the hydrate of a halide. Thus, a total amount of the addition for the practical purpose is about 5% by weight. Further, like in the case of the above-mentioned anhydrides, it is advatageous also in the case of hydrates to combine e.g. the hydrate of a nitrate and the hydrate of a halide in that it is thereby possible to obtain a great effect with small amounts of addition.

If the composition of CaCl₂.6H₂ O containing 2% by weight of each of NH₄ NO₃ and NH₄ Br is represented by molar %, it comprises 90.2 molar % of CaCl₂.6H₂ O, 5.4 molar % of NH₄ NO₃ and 4.4 molar % of NH₄ Br, whereby the total amount of the NH₄ NO₃ and NH₄ Br is as high as about 10 molar %. Likewise, the composition containing 2% by weight of each of KNO₃ and KBr, comprises 92 molar % of CaCl₂.6H₂ O, 4.3 molar % of KNO₃ and 3.7 molar % of KBr.

Now, the present invention will be described in further detail with reference to Examples 7 to 13. However, it should be understood that the present invention is by no means restricted by these specific examples.

EXAMPLE 1

A composition of CaCl₂.6.11H₂ O mixed with 5% by weight of KBr was packed in a high density polyethylene tube having an inner diameter of 40 mm and a length of 1000 mm, and at the same time, 0.01% by weight of BaZrO₃ was added as a nucleating agent. The mixture started to solidify when cooled to 18° C., remained at 21° C. during the solidification and formed a uniform liquid phase at 30° C., and thus repeated the steady and constant phase change.

EXAMPLE 2

A composition of CaCl₂.6.11H₂ O mixed with 5% by weight of NH₄ Br was packed in a high density polyethylene tube having an inner diameter of 40 mm and a length of 1000 mm, and at the same time, 0.01% by weight of BaZrO₃ was added as a nucleating agent. The mixture started to solidify when cooled to 18° C., remained at 21.5° C. during the solidification and formed a uniform liquid phase at 30° C., and thus repeated the steady and constant phase change.

EXAMPLE 3

A composition of CaCl₂.6.08H₂ O mixed with 4% by weight of KNO₃ was packed in the same high density polyethylene tube as in Example 1, and 0.05% by weight of BaTiO₃ was added as a nucleating agent. The mixture started to solidify when cooled to 18° C., and formed a uniform liquid phase at 30° C., and thus repeated the steady and constant phase change.

EXAMPLE 4

A composition of CaCl₂.6.08H₂ O mixed with 2% by weight of NH₄ NO₃ was packed in the same high density polyethylene tube as in Example 1, and 0.05% by weight of BaTiO₃ was added as a nucleating agent. The mixture started to solidify when cooled to 20° C., and formed a uniform liquid phase at 30° C., and thus repeated the steady and constant phase change.

EXAMPLE 5

A composition of CaCl₂.6.11H₂ O mixed with 2% by weight of KNO₃, 2% by weight of KBr and 0.5% by weight of NaCl was packed in the same high density polyethylene tube as in Example 1, and at the same time, 0.01% by weight of BaZrO₃ was added as a nucleating agent. The mixture sealed in the tube started to solidify when cooled to 20° C., maintained the heat generating temperature of 23° C. during the solidification and formed a uniform liquid phase at 30° C., and thus repeated the steady and constant phase change.

EXAMPLE 6

A composition of CaCl₂.6.11H₂ O mixed with 2% by weight of NH₄ NO₃, 2% by weight of NH₄ Br and 0.5% by weight of NaCl was packed in the same high density polyethylene tube as in Example 1, and at the same time, 0.01% by weight of BaZrO₃ was added as a nucleating agent. The mixture sealed in the tube started to solidify when cooled to 15° C., maintained the heat generating temperature of 20° C. during the solidification and formed a uniform liquid phase at 26° C., and thus repeated the steady and constant phase change.

EXAMPLE 7

To CaCl₂.6.1H₂ O (in which at least 0.2% by weight of NaCl was dissolved), 2% by weight of NH₄ NO₃ and 2% by weight of NH₄ Br were added, and 1% by weight of Ca(NO₃)₂.4H₂ O was further added. The mixture was fused to obtain a uniform solution. After adding 0.1% by weight of BaZrO₃ as a nucleating agent, the solution was subjected to a heat cycle test at 30° to 12° C. For the heat cycle test, a sample was placed in a vertical glass tube having an inner diameter of 20 mm (the height of the liquid: about 900 mm), and by means of five thermocouples (placed in a distance of 150 mm), the temperature changes at the respective portions were recorded. As a result, it was found that the steady and constant phase change was repeated for 1200 times or more, and no phase separation of CaCl₂.4H₂ O was observed. The temperature characteristics at the time of the solidification were as shown in Table 3.

EXAMPLE 8

A composition prepared in the same manner as in Example 7 except that instead of Ca(NO₃)₂.4H₂ O, the same amount of Mg(NO₃)₂.6H₂ O was added, was subjected to the same test as in Example 7, whereby the steady and constant phase change was repeated for 1200 times or more and no crystallization of CaCl₂.4H₂ O was observed. The temperature characteristics at the time of the solidification were as shown in Table 3.

EXAMPLE 9

To CaCl₂.6.0H₂ O, 2% by weight of KNO₃ and 2% by weight of KBr were added, and 1% by weight of Ca(NO₃)₂.4H₂ O was further added. The mixture was fused to obtain a uniform solution. After adding 0.1% by weight of BaTiO₃ as a nucleating agent, the solution was subjected to the same heat cycle test as in Example 7, whereby the steady and constant phase change was repeated for 1200 times or more. The temperature characteristics at the time of the solidification were as shown in Table 3.

EXAMPLE 10

A composition prepared in the same manner as in Example 9 except that instead of Ca(NO₃)₂.4H₂ O, the same amount of Mg(NO₃)2.6H₂ O was added, was subjected to the same heat cycle test as in Example 7, whereby the steady and constant phase change was repeated for 1200 times or more, and no crystallization of CaCl₂.4H₂ O was observed. The solidification characteristics were as shown in Table 3.

EXAMPLE 11

A composition prepared in the same manner as in Example 7 except that instead of Ca(NO₃)₂.4H₂ O, 2% by weight of CaBr₂.6H₂ O was added, was subjected to the same test as in Example 7, whereby the steady and constant phase change was repeated for 1200 times or more. The freezing point was 20° C.

EXAMPLE 12

A composition prepared in the same manner as in Example 9 except that instead of Ca(NO₃)₂.4H₂ O, 2% by weight of MgBr₂.6H₂ O was added, was subjected to the same test as in Example 7, whereby the steady and constant phase change was repeated for 1200 times or more. The freezing point was 20° C.

EXAMPLE 13

A composition prepared in the same manner as in Example 7 except that instead of Ca(NO₃)₂.4H₂ O, 1% by weight of MgCl₂.6H₂ O was added, had a freezing point of 21° C., and when subjected to the same test as in Example 7, repeated the steady and constant phase change for 1200 time or more.

Table 3 shows the temperature changes of the compositions at the time of the solidification, wherein Comparative Examples 1 and 2 represent compositions (No. 19 and No. 13) containing anhydrides without containing the fourth additive, and Examples 7 to 10 represent the above-mentioned typical compositions. In the Table, "First" and "Last" represent the temperatures of the portions which solidify first and last, respectively. In both the "First" and "Last", the first mentioned value indicates the temperature at the initiation of the solidification (i.e. the temperature at which the supercooled state was broken) and the last mentioned value indicates the heat generation temperature (i.e. the freezing point) during the solidification.

The differences of 3.5° C. and 3.7° C. in the freezing points of the "First" and "Last" as observed in the respective Comparative Examples where no hydrates were added, were not observed immediately after the respective samples were uniformly mixed, and such differences appeared as the phase change was repeated. Accordingly, it is considered that a concentration gradient in the vertical direction was formed in the solution during the repeated phase change. In fact, the average values of the freezing points of the "First" and "Last" substantially correspond to the melting points of the respective samples. The term "concentration gradient" is used in this sense. However, when 1% by weight of Ca(NO₃)₂.4H₂ O or Mg(NO₃)₂.6H₂ O is added to such a composition, the freezing points of the "First" and "Last" agree to each other with the difference being within 1° C., and at the same time, the temperature at the initiation of the solidification of the "Last" increases by a few °C., thus indicating that the supercooling is improved so that the solidification is facilitated. It is considered that in a CaCl₂.6H₂ O-anhydride system, the interaction between CaCl₂.6H₂ O as the main component and the anhydride as the additive is somehow poor, whereas when a small amount of a hydrate is added, the hydrate somehow serves as an intermediary for the main component and the additive. Namely, it may be said that the effect of the hydrate as the fourth additive is not only to prevent the formation of the concentration gradient but also to improve the solidification characteristics.

                  TABLE 3                                                          ______________________________________                                         Temperature changes at the time of solidification                              ______________________________________                                         Comparative                                                                    Example 1 (No. 19)                                                                         Example 7      Example 8                                           NH.sub.4 NO.sub.3 --NH.sub.4 Br                                                            Ca(NO.sub.3).sub.2.4H.sub.2 O                                                                 Mg(NO.sub.3).sub.2.6H.sub.2 O                       2% by weight each                                                                          0.1% by weight 0.1% by weight                                      First  Last     First     Last   First  Last                                   (°C.)                                                                          (°C.)                                                                            (°C.)                                                                             (°C.)                                                                          (°C.)                                                                          (°C.)                           ______________________________________                                         18.6;  14.5;    19.5;     16.9;  19.5;  17.6;                                  20.0   23.5     20.5      19.8   20.7   21.0                                   ______________________________________                                         Comparative                                                                    Example 2 (No. 13)                                                                         Example 9      Example 10                                          KNO.sub.3 --KBr                                                                            Ca(NO.sub.3).sub.2.4H.sub.2 O                                                                 Mg(NO.sub.3).sub.2.6H.sub.2 O                       2% by weight each                                                                          0.1% by weight 0.1% by weight                                      First  Last     First     Last   First  Last                                   (°C.)                                                                          (°C.)                                                                            (°C.)                                                                             (°C.)                                                                          (°C.)                                                                          (°C.)                           ______________________________________                                         20.2;  15.1;    19.8;     17.5;  19.7;  18.0;                                  20.5   23.2     20.7      20.2   20.5   20.2                                   ______________________________________                                    

Further, when a small amount of the hydrate is added, no substantial decrease in the heat of fusion is observed, and the decrease is not greater than 2 cal/g especially when a hydrate of a halide is added, the heat of fusion may be calculated by summing up the products obtained by multiplying the contents of the respective components by the respective heat of fusion in accordance with the approximation of an ideal solution.

The Examples given above illustrate the cases wherein the hydrates are added individually. However, when two or more different hydrates are added in combination, particularly in the case of the combination of the hydrate of nitrate with the hydrate of a halide, a further improved effect is obtainable by the synergistic effects of the plurality of hydrates.

As described in the foregoing, it is possible not only to prevent the formation of the concentration gradient and the supercooling but also to obtain a heat storage material having improved solidification characteristics and phase change stability by using CaCl₂.6H₂ O which contains (a) an anhydrous bromide, (b) an anhydrous nitrate and (c) at least one hydrate selected from the group consisting of hydrates of a chloride, a bromide and a nitrate. Further, by the addition of the hydrate, it is possible to control the melting point of the heat storage material within a range of a few °C. to obtain a composition suitable for the particular purpose. 

I claim:
 1. A heat storage material which comprises CaCl₂.6H₂ O, as a main component, an amount effective to decrease the melting point thereof of at least one anhydrous component selected from the group consisting of NH₄ Br and KNO₃ and mixtures thereof with at least one of KBr and NH₄ NO₃, and up to an amount not exceeding the saturation point of the anhydrous NH₄ Br or KNO₃ in the liquefied heat storage material at 30° C. and not exceeding a concentration of 10 percent by weight, and 0 to 10% of at least one hydrate selected from the group consisting of a hydrate of a chloride other than of calcium, a bromide and a nitrate.
 2. The storage material according to claim 1, wherein the anhydrous component includes NH₄ Br.
 3. The heat storage material according to claim 1, wherein the anhydrous component includes KNO₃.
 4. The heat storage material according to claim 2, wherein the anhydrous component includes NH₄ NO₃ or KNO₃.
 5. A heat storage material which comprises CaCl₂.6H₂ O, as a main component, containing (a) an anhydrous bromide, (b) an anhydrous nitrate and (c) at least one hydrate selected from the group consisting of a hydrate of a chloride of other than calcium, a bromide and a nitrate, the total of (a) and (b) being an amount effective to decrease the melting point of CaCl₂.6H₂ O and up to an amount not exceeding the saturation point of the same in the liquefied heat storage material at 30° C. and not exceeding a concentration of up to 10% by weight, and the amount (c) being effective to diminish supercooling but not more than 10% by weight of the composition.
 6. The heat storage material according to claim 5, wherein the anhydrous bromide is NH₄ Br or KBr.
 7. The heat storage material according to claim 5, wherein the anhydrous nitrate is NH₄ NO₃ or KNO₃.
 8. The heat storage material according to claim 5, wherein the hydrate of a chloride is MgCl₂.6H₂ O.
 9. The heat storage material according to claim 5, wherein the hydrate of a bromide is at least one hydrate selected from the group consisting of CaBr₂.6H₂ O and MgBr₂.6H₂ O.
 10. The heat storage material according to claim 5, wherein the hydrate of a nitrate is at least one hydrate selected from the group consisting of Ca(NO₃)₂.4H₂ O and Mg(NO₃)₂.6H₂ O.
 11. The heat storage material according to claim 3 wherein the anhydrous component includes KBr.
 12. The heat storage material according to claim 1 which includes a nucleating amount of a nucleating agent. 